This invention relates generally to an optical coupler for coupling a first beam and a second beam having different wavelengths to each other, and more particularly to an optical coupler suitably used to couple a signal beam and a pumping beam having proximate wavelengths to each other and introduce the beam obtained by such coupling into an Er doped fiber.
In an optical fiber communication system which has been put into practical use, a repeater is interposed at each predetermined distance in order to compensate for attenuation of an optical signal caused by loss in an optical fiber. Such repeater is constituted such that an optical signal is converted into an electric signal by a photodiode, and the electric signal is amplified by an electronic amplifier and is then converted back into an optical signal by a semiconductor laser or the like, whereafter the optical signal is sent out into an optical fiber transmission line. If such optical signal can be amplified in low noises while it remains in the form of an optical signal, then the repeater can be reduced in size and economized.
Thus, investigation of optical amplifiers which can directly amplify an optical signal is being proceeded extensively. Optical amplifiers which are made objects of investigation are divided roughly into three types including (a) a first type which includes a combination of an optical fiber doped with a rare earth element (Er, Nd, Yb and so forth) and a pumping beam, (b) a second type which includes a semiconductor laser doped with a rare earth element, and (c) a third type which makes use of a non-linear effect in an optical fiber such as an induced Raman amplifier or an induced Brillouin amplifier.
An optical amplifier of the first type (a) which includes a combination of an optical fiber doped with a rare earth element and a pumping beam among the three types listed above has superior features that it has no polarization dependency, that it produces a comparatively small amount of noises and that the coupling loss is low. Therefore, it is anticipated that an optical fiber of the type (a) enables an increase of the repeater to repeater distance in an optical fiber transmission system and further enables distribution of optical signals to a large number of stations.
An Er doped optical fiber which is doped with Er as a rare earth element is commonly used for the amplification of the 1.55 .mu.m band which is used in optical fiber communications.
FIG. 1 illustrates a principle of optical amplification by a rare earth element doped optical fiber. Reference numeral 2 denotes an optical fiber constituted from a core 4 and a clad 6, and Er (erbium) is doped in the core 4. If a pumping beam of a wavelength of, for example, 1.48 .mu.m is introduced into such Er doped optical fiber 2, then Er atoms are excited to a higher energy level by the pumping beam. If a signal beam of another wavelength of, for example, 1.55 .mu.m is introduced to such Er atoms in the optical fiber 2 which are excited to a higher energy level in this manner, the Er atoms drop to a lower energy level, whereupon stimulated emission of light takes place so that the power of the signal beam is increased gradually along the optical fiber, thereby effecting amplification of the signal beam.
FIG. 2 shows general construction of a conventional Er doped optical fiber amplifier based on the principle of amplification described just above. Reference numeral 10 denotes a polarized beams coupler, and a first pumping beam having a horizontal polarization plane and a second pumping beam having a vertical polarization plane are introduced into the polarized beams coupler 10 by way of a pair of optical fibers 11 and 12, respectively. The first and second pumping beams are combined with each other by and emitted from the polarized beams coupler 10. The first and second pumping beams have, for example, an equal wavelength of 1.48 .mu.m. The polarized beams coupler 10 includes a polarized beams coupling prism unit 16 of a construction wherein a polarized beams coupling film 15 is sandwiched between a pair of triangular prisms 13 and 14.
The first pumping beam introduced in from the optical fiber 11 and having a horizontal polarization plane is collimated by a collimator lens 17 and then is introduced into the polarized beams coupling prism unit 16, in which it passes through the polarized beams coupling film 15 as it is. On the other hand, the second pumping beam introduced in from the optical fiber 12 and having a vertical polarization plane is collimated by another collimator lens 18 and is then introduced into the polarized beams coupling prism unit 16, in which it is reflected into the same light path as the transmitted light path of the first pumping beam by the polarized beams coupling film 15. Accordingly, the emergent beam from the polarized beams coupling prism unit 16 is a combination of the first pumping beam and the second pumping beam, and the pumping beams obtained by such coupling is condensed by a lens 19 and introduced into an optical fiber 20. Another optical fiber 21 is connected to the optical fiber 20 by way of a connecting adapter 22, and the first and second pumping beams transmitted by way of the optical fibers 20 and 21 are introduced into an optical wavelength multiplexer 23, in which they are wavelength multiplexed with a signal beam of, for example, the 1.55 .mu.m band transmitted thereto along an optical fiber 28.
The optical wavelength multiplexer 23 includes an optical wavelength multiplexing prism 26 which in turn includes a triangular prism 24 and an optical wavelength multiplexing film 25 in the form of a dielectric multi-layer film or the like formed on an inclined face of the triangular prism 24. The first and second pumping beams from the optical fiber 21 are collimated by a collimator lens 27 and are then introduced into the optical wavelength multiplexing prism 26, in which they are transmitted through the optical multiplexing film 25, whereafter they are emitted from the optical wavelength multiplexing prism 26. On the other hand, the signal beam of the wavelength of, for example, 1.55 .mu.m which has been transmitted along the optical fiber 28 is collimated by a collimator lens 29 and is then introduced into the optical wavelength multiplexing prism 26, in which it is reflected by the optical wavelength multiplexing film 25. Consequently, the signal beam is emitted into the same light pass as the transmitted light path of the pumping beams. The signal beam and the pumping beams wavelength multiplexed in this manner are condensed by a lens 30 and introduced into an Er doped optical fiber 31 in which amplification of the signal beam takes place based on the principle described hereinabove.
The reason why a first pumping beam and a second pumping beam are coupled to each other by the polarized beams coupler 10 in the construction of the conventional Er doped optical fiber amplifier is that, when the output power of a semiconductor laser (LD) which emits a pumping beam is not sufficiently high, two semiconductor lasers are used in order to assure a pumping beam of a high output power. In case the output powers of the semiconductor lasers are sufficiently high, only one of two pumping beams can be used, thereby employing a redundant structure.
In the Er doped optical fiber amplifier, a laser beam of the wavelength of 1.55 .mu.m band and another laser beam of the wavelength of 1.48 .mu.m are employed as a signal beam and a pumping beam, respectively, as described hereinabove. Where the difference in wavelength between a signal beam and a pumping beam is small in this manner, if the angle of incidence of the signal beam to an optical wavelength multiplexing film is not smaller than 45 degrees, wavelength multiplexing of the signal beam and the pumping beam cannot be achieved sufficiently. Accordingly, there is a problem that such signal beam cannot be introduced into an optical wavelength multiplexing prism perpendicularly or parallelly to the light path of the pumping beam, and accordingly, the arrangement of optical parts including an optical wavelength multiplexing prism becomes complicated.